This tart condiment is a good way to use up tomatoes before the first frost.
Wash and core tomatoes. Cut them into 1/4-inch-thick wedges. Peel onion and cut into 1/4-inch-thick wedges. Combine tomato and onion wedges, raisins, vinegar, and 1/4 cup water in a large saucepan; cook over medium-high heat until tomatoes begin to break down and release their juices, about 10 minutes.
Reduce heat to medium-low, sprinkle tomatoes with sugar, mustard, and salt, and stir well to combine. Continue cooking until tomatoes are almost completely broken down and juices are thick and syrupy, about 1 hour. Remove from heat and let cool completely before serving. Cover and refrigerate for up to 1 week.